During your Main Phase: You can Fusion Summon 1 Fiend Fusion Monster from your Extra Deck, using monsters from your hand or field as material. You can only use this effect of "Dark Contract with the Swamp King" once per turn. If Summoning a "D/D" Fusion Monster this way, you can also banish monsters from your GY as material. Once per turn, during your Standby Phase: Take 1000 damage.